1. Field of the Invention
The present invention relates to freshness keeping sheets and also to a method for keeping vegetables and fruits fresh by use of the freshness keeping sheets. More specifically, the present invention relates to freshness keeping sheets obtained by adding palladium chloride having oxidation catalytic activity to activated carbon and molding the carbon into sheets, and to a method for keeping fresh, by use of the sheets, vegetables and fruits preserved in a box or a section of refrigerator for storing them.
2. Description of the Prior Art
The use of a catalyst comprising palladium chloride carried on activated carbon for oxidation of carbon monoxide or ethylene is known. In this case, particulate or granular activated carbon has been mostly used as a carrier [Nihon Kagakukaishi, p.1092-1087 (1972), Japanese Patent Publication No. 10776/1985, Japanese Patent Application Laid-open No. 55147/1981]. The present applicant has applied for patent of an activated carbon carrying palladium chloride (Japanese Patent Application No. 10776/1985 and Japanese Patent Application Laid-open No. 55147/1981). In the applications, the activated carbon to carry palladium chloride is granular and should hence be used only as packed in small bags or columns, which causes problems when used in a narrow space such as in refrigerators.
Also known is a process of permitting a honeycomb shaped activated carbon to carry palladium chloride. With this process, it is difficult to permit the honeycomb activated carbon as the carrier, having a low strength and small specific surface area, to carry a large amount of palladium chloride, and hence a high-activity palladium chloride-carrying activated carbon cannot be obtained. There are still other problems, i.e. honeycomb activated carbon is expensive and the palladium chloride-carrying honeycomb activated carbon requires forced ventilation with a fan or the like.
Let us consider the case where a freshness keeping system is, for example, used for a refrigerator. If the system requires, like in the case of conventional deodorizing systems, circulation of whole gases present in the refrigerator, an additional fan should be mounted and, at the same time, the catalytic activity for decomposing ethylene will sometimes markedly decrease due to adsorption of toxic gases generated from other foods. In order to achieve the object of keeping freshness more effectively, it is better to remove ethylene present in the very local part where it generates, such as a vegetables and fruits section in the refrigerator. This object cannot be achieved by use of the known granular palladium chloride-carrying activated carbon. This is because granular activated carbon, which should be used while being packed in a column or container, can hardly be used effectively in such a narrow place as in the refrigerator.
There has been desired, in order to remove ethylene effectively even in such narrow places, development of a shaped freshness keeping sheet obtained by molding a palladium chloride-carrying activated carbon having high performance and a large surface area, without deteriorating its performance.
A conventional process for molding activated carbon into sheets uses a liquid resin binder. This process causes the liquid to cover the pores of the activated carbon, thereby decreasing the performance markedly and preventing the activating carbon from exhibiting a high adsorbing capability inherent thereto.
One may attempt to prepare an oxidation catalyst by permitting a sheet-shaped activated carbon to carry palladium chloride. This method limits the performance of the sheet-shaped activated carbon, so that an activated carbon having a large specific surface area cannot be used. This is because activation treatment to obtain a high-performance activated carbon sheet causes the problems of crack generation, strength decrease, deformation and the like. Thus, the activated carbon sheet used for this purpose can only possess a specific surface area of about 1,000 m.sup.2 /g at most and in general about 1,000 m.sup.2 /g.